I've just made this account so I can't post on the Troubleshooting forums. I bought a 2007 RX8 192ps about a month ago. The car drives well, compression was tested a few months back and scored 7.2, 7.4, 7.7 and 7.2, 7.3, 7.7 at 290rpm. The car was serviced when I bought it and spark plugs were in good condition so weren't changed. The car idles kind of rough when warmed up, it stays around 850-900 and drops to 800-775 for a second then goes back up (although my brothers rx7 idles similarly). The car also shakes when it does that but doesn't stall. After not driving it for 4 days and the weather is relatively cold, 6-9 degrees, I started it up. IT started at 2.5k and a couple of seconds later dropped to 1.2k for a split second, then it went back up 2.5k and went through its normal warm-up routine. I also changed the lighter fuse as it was blown 6 days ago and from then there has been a high-pitched whistle that gets louder as you rev and cuts off when the clutch is down. It sounds kind of like a turbo whistle but more reminiscent of air going through a small hole like an old-school kettle. I'm suspecting a vacuum leak perhaps but I'm not sure. Any advice would be appreciated.

 

Is the A/C on? The compressor cycling on will cause idle to drop, this is normal. Whistling and hunting is not though. I think you're on the right track with a vacuum leak, you can confirm by either searching for where it could be or using an OBD scanner to check fuel trims and MAF rate on a warm idle. Fuel trims should be 0 or near. If they higher than 10 or so, at 800rpm idle, extra air is getting in somewhere.

 

I am currently hunting why my car has inconsistent idle as well. I just came back inside from replacing the coils with my brand new BHR coils, but still no joy. the car actually stalled on me on startup, like it did before when it threw the code for the SSV being stuck. But even when the car settled out and the valve closed, the car still idles rough and the exhaust note does not sound consistent and healthy. The new coils did fix the exhaust smelling rich and the extra hot exhaust, so that was fixed. But the car still idles odd...

 

I replaced my plugs, wires, and coils (BHR as well) and found the same idle issue. NVRAM reset cleared it up.

 

Hunting/rough idle can be caused by a few factors.

• MAF - simple enough to clean with MAF cleaner
• Timing - Mainly checking that your E-shaft sensor is clean and then putting your key in the "on position" then doing the 20 brake stomp reset.(your oil pressure gauge will bounce if done correctly)
• Poor ignition/spark - The Rx8 coils are notorious for having issues even in low mile cars. Sparkplugs are easy enough to check and if they are gunked up. Note if you get a new set of plugs and run into similar issues within a month or so then you have to start looking elsewhere.
• Compression test - at a certain point if you manage to run your engine to its deathbed, compression will get so low that you will see a hunting idle and stalling. Typically you will see hotstart issues beforehand. [IMO at around 70k miles you should rebuild the renesis motor to save it from unnecessary damage and I'd be willing to bet your rotors will not have their Apex Slots bent outwards due to worn down seals]
• Vacuum leaks - the rx8 has a lot of vacuum lines and places to check. Plenty of diagrams are available on the forum. Generally you will find most of the lines using a 10mm to take apart the upper intake. There is one line that runs down to the bottom of the lower intake manifold, just above the exhaust manifold. Which can heavily impact idle since it directly reaches into the primary ports.